I changed System>Advanced>Miscellaneous>PHP Settings>Memory Limit from 128 to 256, rebooted and the banner is gone.
Presumably this has resolved it. I have a second 1100 which I'll repeat the update and see if the error recurs. If so, I'll see if the above fix resolves it as well.
Seems the update process, at least, should make this adjustment instead of leaving a mess requiring the customer to trouble-shoot.
In the long run, as I've observe elsewhere before, the SG1100 seems to be under-resourced for pfSense's modern versions.